Control of pressure in drying apparatus such as centrifugal evaporators is important for 3 reasons:                (i) To allow controlled reduction of the chamber pressure in order to prevent bumping        (ii) To limit the ultimate chamber pressure so as to prevent solvent from freezing during evaporation. This is particularly important when evaporating water. Evaporation from the solid phase (sublimation) is much slower than evaporation from the liquid phase.        (iii) To limit the ultimate chamber pressure in order to maximize solvent recovery. This is particularly important with volatile solvents, typically those that would boil at −30° C. or below at the ultimate pressure achieved in the chamber.        
There are two methods in common use for controlling pressure:
In a first method which has been used, possibly for 20 years or more to control pressure in Freeze Dryers, an adjustable bleed valve is connected to the drying chamber, and in use a controlled amount of nitrogen or air is steadily bled into the chamber while continuously vacuum pumping the chamber. This method allows objectives (ii) and (iii) to be achieved but objective (i) proved very difficult to achieve when the object is to secure an increasing vacuum ramp. Proportional control of the bleed valve over a large dynamic range would be required to achieve all three requirements listed above.
A second method is described in U.S. Pat. No. 5,137,604. Here the pump is isolated from the chamber in response to a signal proportional to the pressure within the drying chamber. This method works well with pumps that: a) do not suffer from condensation of vapors internally (e.g. scroll pumps, vane pumps with gas ballasting), b) would shed oil if operated open to atmosphere for extended periods (vane pumps). This method has some disadvantages: the isolation valve must be solvent resistant and as such is a relatively expensive component because the bore must be large. When used to control the pressure with pumps that suffer from performance loss when solvents condense internally it is found that performance decreases significantly over the duration of an evaporation run. Pump performance is only recovered when a significant flow rate of gas is once again established through the pump, to force out the condensed solvent. Multi-head diaphragm pumps, which are commonly used for evaporation equipment are affected badly by solvents condensing internally. An additional problem which has been found due to liquid solvent entrapment within the pump, is premature degradation of the pump components especially elastomeric seals within the pump.
Apparatus for solvent removal will be referred to as drying apparatus. Drying apparatus can include centrifugal evaporators, freeze driers, and rotary evaporators in which a film of solvent is maintained over the inside surface of a flask by rotating it in a warm fluid.